1. Field of the Invention
The present invention relates to a traffic control method and system for controlling cell traffic in an asynchronous transfer mode (ATM) communication network.
2. Description of Background
In recent communications, many types of communication media such as voice, information data, facsimile, and picture are mixed, while communication networks are independently formed for each medium or each use in conventional communication networks. Therefore, those networks are inadequate for reducing communication costs by unifying communication facilities and providing flexible networks.
As mentioned above, recently the need has developed for communications for various purposes so that the unification of communication networks is required to efficiently provide communication service. To satisfy that requirement, new transmission techniques, which differ from conventional line or packet switching, have been researched and developed.
In particular, an asynchronous transfer mode (ATM) has attracted considerable attention because the ATM is one transmission mode for efficiently embodying high speed and wide band communication networks. In the ATM, both techniques of recent line switching and packet switching are unified and many pieces of information such as voice, data, and picture are divided into information units to be transmitted and switched at 150 Mbits per second. The unit is called a "cell" and has a prescribed length.
FIG. 1 schematically shows an ATM communication network. In an ATM communication network 34, a plurality of terminals 31, 32, 33, 35, 36, and 37 are connected and cell transmission is executed between the terminals through the ATM communication network 34.
In this case, no physical restriction such as the allocation of time slots is imposed on each terminal.
In the ATM communication network 34, exchanges with switching functions are provided to transmit a sequence of cells between the terminals through one of physical transmission channels. As mentioned above, because no physical restriction such as the allocation of timing for submitting cells to the network is imposed on each terminal, there is a possibility that the cells are simultaneously transmitted to the same physical transmission channel from the terminals 31, 32, 33, 35, 36, and 37. Therefore, the exchange is provided with a switch for switching the cells and a buffer for temporarily storing the cells to realize the switching function. The buffer has a certain capacity to store the cells transmitted from the terminals 31, 32, 33, 35, 36, and 37. To store a large number of cells, not only the size of the equipment for realizing the switching function is too large but also the cost of the equipment is too high.
Therefore, practically, the buffer for storing the cells transmitted from the terminals 31, 32, 33, 35 36, and 37 is reduced in size to a certain extent. This means that the cells transmitted from the terminals 31, 32, 33, 35, 36, and 37 must reliably be stored by utilizing the reduced buffer capacity. Therefore, when the cells are intensively transmitted from the terminals 31, 32, 33, 35, 36, and 37 to the buffer, some cells cannot be stored in the buffer because the buffer capacity is insufficient. As a result, the buffer overflows and some cells are lost.
To cope with this situation, a method has been implemented. Whereby, because the buffer is utilized in common by the terminals 31, 32, 33, 35, 36, and 37, each terminal is assigned a prescribed buffer capacity. Therefore, prior to the communication, traffic characteristics such as a minimum cell arrival interval and a maximum cell arrival frequency are declared by each terminal. That is, each of the cells provided from each of the terminals is transmitted to the ATM communication network 34 in accordance with the minimum cell arrival interval. Also, a sequence of cells provided from each of the terminals is transmitted to the ATM communication network 34 in accordance with the maximum cell arrival frequency. Thereafter the required buffer capacity for each terminal is calculated to satisfy a prescribed transmission quality based on the declared traffic characteristics.
Thereafter, when the summation of the buffer capacities of all terminals does not exceed the buffer capacity of the provided buffer, the communication of the terminal is admitted to the network 34. On the other hand, when the summation of the buffer capacities required by terminals exceeds the capacity of the provided buffer so that a summed buffer capacity more than the capacity of the provided buffer is required to perform the communication, the communication is rejected. If possible, the communication may be suspended until the communication of a part of terminals is terminated, and it can be admitted. As an example of the assignment of the buffer capacity, the minimum cell arrival interval or the maximum cell arrival frequency is negotiated between the communication network 34 and each terminal prior to the communication so that the buffer capacity is logically assigned for each terminal in the communication network 34 by considering the minimum cell arrival interval and the maximum cell arrival frequency. The minimum cell arrival interval means that the cells are transmitted from a certain terminal at prescribed intervals which are each within the minimum cell arrival interval. The maximum cell arrival frequency means the upper limitation of the number of cells transmitted from a certain terminal within a prescrived period of time.
However, in the above assignment of the buffer capacity, there is a case that a larger number of cells than the negotiated number of cells are transmitted from a certain terminal intentionally or by a failure. To cope with this situation, cells flow in the ATM communication network 34 in accordance with inlet traffic characteristics determined by intervals of the cells and the frequency of the cells is monitored at the inlet of the ATM communication network 34 so that the inflow of the cells exceeding prescribed traffic characteristics determined by considering the minimum cell arrival interval and the maximum cell arrival frequency allowed to the cells transmitted to the ATM communication network at the 34 is regulated and, for example, rejected. This operation is called "policing", or UPC (Usage Parameter Control).
On the other hand, the inlet traffic characteristics are varied to outlet traffic characteristics determined by intervals and the frequency of the cells flowing out at the of the ATM communication network 34 because the cells are temporarily stored in the buffer to prevent cell loss due to simultaneous arrival of the cells. Therefore, the outflow of the cells in accordance with the outlet traffic characteristics must be controlled and corrected to the a prescribed cell flow in accordance with the prescribed traffic characteristics at the outlet of the ATM communication network 34 when the cells are transmitted to another ATM communication network. Then, the controlled cells are transmitted through the other ATM communication network to a prescribed terminal. This operation is called "traffic shaping". Policing and traffic shaping are implemented to make the sphere of the responsibility clear when a first communication network managed by a first company is connected with a second communication network managed by a second company so that the cells are transmitted through the first and second communication networks.
However, in the case where a sequence of cells is are transmitted in an ATM communication network which is subdivided into many systems, the traffic characteristics of the sequence of cells are changed at the outlet of each system because a switching operation and a multiplexing operation are performed at each subdivided system. The change of the traffic characteristics is described with reference to FIG. 7 in detail. As shown in FIG. 7, a sequence of cells C1 is transmitted between terminals through a physical transmission channel A at a cell arrival interval of 10 unit length. is, the cells C1 are transmitted at traffic characteristics determined by the cell arrival interval of 10 unit length. Also, another sequence of cells C2 is transmitted between the terminals through another physical transmission channel B at a cell arrival interval of 10 unit length. That is, the cells C2 are transmitted at the same traffic characteristics as those of the cells C1. When both the physical transmission channels A, B are combined at a system such as an exchange to a physical transmission channel C, a sequence of cells transmitted through the physical transmission channel C is not transmitted at traffic characteristics determined by a cell arrival interval of 5 unit length but transmitted at traffic characteristics determined by a cell arrival interval of 1 unit length. Therefore, the traffic characteristics of the cells is considerably changed at each of the systems. The influence of the change of the traffic characteristics extends to the downstream subdivided systems arranged at the outlet direction of the ATM communication network.
Therefore, an extra amount of control is required to transmit the cells while predicting the above change in advance.
Moreover, when the traffic characteristics are extremely changed at a system due to a failure of the system, it is difficult to specify the location of the system relating to the failure in the ATM communication network.
As mentioned above, in the case where policing and traffic shaping are performed at the inlet and outlet portions of the ATM communication network and the cells are transmitted through the ATM communication network which is subdivided into many systems, the traffic characteristics of the cells are changed at the outlet of each system. Therefore, there is a drawback that the influence of the change of the traffic characteristics extends to the downstream subdivided systems arranged at the outlet direction of the ATM communication network.
Moreover, to cope with the above drawback, an extra amount of control is required to transmit the cells while predicting the above change in advance.
Further, there is another drawback in that it is difficult to specify the location of a failure system when the traffic characteristics are extremely changed somewhere in the ATM communication network.